Google Chrome DLs quicker than Firefox IMO
Unless you care to cough up evidence to support such claims, it seems ridiculous. Page loading speed in Chrome is faster due to the web engine, but that has no impact on downloads.
Anyway, two things to try:
1. http://support.mozilla.com/en-US/kb/unable+to+download+or+save+files#Disable_virus_scanning_in_Firefox_preferences
2. I had a similar issue (though not identicle) in Opera, nothing fixed it except a complete clean install (by default browsers, including Firefox, leave a lot of stuff behind in the profile folder (Appdata in Windows Vista/7). Delete all traces of it and run CCleaner to pick up any leftover registry entries and then do a clean install.